1. Field of the Invention
The present invention relates to a process and apparatus for heat treating a copper film formed on a ceramic body. More specifically, the present invention relates to a process and apparatus for heat treatment of a copper film formed as an electrode or a conductive device in a ceramic electric circuit component including a ceramic body of such as a dielectric, insulating, resistive, semiconductive or the like material.
2. Description of the Prior Art
One example of a ceramic electric circuit component of interest to the present invention is a ceramic capacitor. The electrode of a ceramic capacitor is generally formed using silver of high conductivity. Such silver electrode is formed by coating a silver paste on a ceramic body and by baking the same. However, with the recent rise of the cost of silver material, the ratio of the cost of such silver electrode to the total cost of a ceramic capacitor has increased. Thus, employment of such silver electrode has become a cause of the rise of the total cost of ceramic capacitors.
As a result of the foregoing, attempts have been made to development of an inexpensive electrode. As one approach, various studies were made as to a method for forming a metallic film, such as an electroless plating process, a vacuum evaporation process, a sputtering process, an ion plating process, and the like. As another approach, study was made as to employment of inexpensive metal as a substitute for silver as an electrode.
The first approach employed was a nickel plated electrode formed by means of an electroless plating process. A nickle plated electrode was successful to some extent as an inexpensive electrode substitute for a silver electrode. However, it was observed that employment of a nickel plated electrode as an electrode of ceramic capacitors involves the following problems. More specifically, the resistivity of a nickel electrode per se is 7.24.times.10.sup.-6 .OMEGA..cm, which is higher than that of silver being 1.62.times.10.sup.-6 .OMEGA..cm. Accordingly, a problem is caused that a frequency characteristic is degraded in the high frequency region. Another problem is that solderability of a nickel plated electrode is poor. Furthermore, another approach was attempted in which the whole surface was coated with a solder layer in order to decrease the resistivity of the nickel plated electrode. However, in coating the whole surface of the electrode with a solder layer a large amount of flux which is active must be used. Accordingly, it is necessary to cleanse the electrode to remove unnecessary flux after soldering. Furthermore, although the electrode portion is soldered by dipping the same in a solder tub, for example, such a process causes stress in the ceramics, so that a crack is liable to be caused in the ceramics.
As a further approach, attempts were made to develop an inexpensive electrode which can be substituted for the above described nickel electrode. A copper plated electrode applied by means of a copper electroless plating process was tried. However, it was observed that a serious obstacle is involved in a copper electroless plated electrode. Particularly, a copper plated electrode formed by an electroless plating process has a large resistivity as compared with that of an electrode formed from a bulk of copper. A similar phenomenon was also observed when a copper electrode was formed by a vacuum evaporation process, a sputtering process, an ion plating process and the like as well as when a copper electrode was formed by an electroless plating process. Accordingly, a copper film formed by the above described various processes requires additional process to exhibit the characteristics of a copper bulk by achieving metallization, dense formation, enhancement of adhesiveness and stabilization. Usually, heat treating is employed to that end.
However, the copper film reacts very actively with oxygen on the occasion of heat treatment, so that the film is oxidized even with a slight amount of oxygen. As a result, a blue oxide film is quickly formed on the surface of the copper film. Conventionally, it was necessary to fully remove oxygen from a heat treating furnace (such as the oxygen absorbed on the furnace wall of the heat treating furnace) using a large amount of nitrogen for a long period of time before carrying out heat treatment of a copper film. In addition, it was necessary to pay careful attention to ensure that an oxygen gas is not be mixed in an ambient gas. The present invention can eliminate the above described problems.